The present invention relates to a magnetic filter with permanent magnets. A particular application thereof is to filtering corrosive products in the primary circuit of nuclear reactors.
It is known that the water which circulates in the primary circuit of a nuclear reactor generally contains ferromagnetic impurities. These impurities generally comprise magnetite and ferrites, products which are relatively insoluble in water and which are therefore transported through the circuit in the form of fine suspended particles.
The ferromagnetic nature of these particles is utilized to eliminate them from the circuit, for which purpose so-called magnetic filters are used. These filters essentially comprise a casing made from non-magnetic material, filled either with a magnetizable lining or with permanent magnets. The magnetizable lining is generally constituted by a bed of balls placed within a winding. The permanent magnets may either comprise magnetic pieces associated with steel grids or a system of multipolar magnetic bars kept spaced by non-magnetic spacers.
These filters function in the following manner. When a fluid containing ferromagnetic impurities passes through the volume containing the magnetized pieces (balls or magnets), the impurities are transported from areas with a weak magnetic field into areas with a strong magnetic field, i.e. towards the magnetic poles of said pieces and become attached thereto.
The filter of the present invention belongs to the second category, i.e. that in which the magnetized pieces are permanent magnets. Interest in this type of construction has been renewed in view of the fact that the materials developed for forming the magnets now make it possible to obtain large fields even under very difficult conditions and specifically at temperatures of about 300.degree. C., such as occur in the primary circuit of a nuclear reactor. Thus, it is now possible to produce permanent magnets which even at this high temperature produce a magnetic field, whose intensity reaches that which was previously obtained with traditional materials, but only at ambient temperature. These advances in connection with magnetized materials increase the efficiency of filters with permanent magnets for various reasons. The fluidity of the water, which is the opposite to its dynamic viscosity, is much higher at 300.degree. C. than at 20.degree. C. (by a factor of approximately 12), whilst the variation in the density of the water and its suspended particles is, with respect to this factor, respectively low and minute. However, the displacement speed of a particle in a flow of water under the action of magnetic forces is precisely proportional to the fluidity of the water, so that the movement of the particles towards the poles is facilitated and the collection of impurities is improved.
Magnetic filters incorporating permanent magnets are already known and reference can be made in this connection to the article by SPILLNER, published in the Journal BRENNSTOFF WARMEKRAFT, 1969, 8, pp. 401-409. Such filters comprise multipolar bars radially fixed around a shaft disposed on the axis of a cylindrical casing. Each bar comprises an end to end assembly of small magnets, whose poles face one another. These magnets are kept spaced by non-magnetic inserts.
Due to this arrangement, the retention capacity of the different magnets is not used to the full, because the magnets tend to act more via their side wall rather than their pole faces. Furthermore, in such an arrangement, the zone towhich the ferromagnetic particles are attached rapidly assumes large dimensions, even in the case of a limited weight of the retained material, which increases the distance of the active areas from the poles and rapidly reduces the action of the magnetic forces on either the still suspended particles or on the already retained particles. The latter are in particular then more easily torn away, if there is a variation in the flow rate in the filter and are then resuspended in the water.